1. Field of the Invention
The present invention relates to determining the position and diameter of the focal point of a laser beam. More specifically, the field of the invention is of an apparatus and method for determining the position and diameter of the focal point of a laser beam, specifically for use in material processing with a high intensity laser beam.
2. Prior Art
The use of laser beams in material processing is well-known. Welding, cutting, boring, chip removal, hardening, and surface treatment are all applications which beneficially utilize a laser.
In all these applications, the laser beam introduces a specific and thus concentrated application of heat energy to a workpiece. Machine workpieces can thereby be formed with little distortion. One example of such a use of a laser beam is found in the German application P 3,925,646, which discloses a process and a device for the erosion of material by means of a laser beam. The German application '646 discloses using the laser beam to heat a specific material volume to an evaporated or molten state, and then blowing away the material with a jet of gas.
In order to optimally use the properties of the laser beam, the position of the focal point and its diameter must be known precisely, especially in the cases of machining workpieces and material. Both the focal point and diameter of the laser beam are determined by the properties of the laser and by the properties of the resonator and transfer lenses used with the laser beam. The aforementioned properties change at high beam output intensities according to the condition and the load on the lenses. As a result, a laser focal point must be determined at the output intensity contemplated for the particular application. Since the focusing of high intensity beams, e.g., CO.sub.2 heavy-duty lasers, results in very high output intensities on the order of 10.sup.3 W/mm.sup.2 (watts per square millimeter) or more, accurately ascertaining the necessary measurements on the laser focal point is relatively problematic. Output intensities on that order will rapidly destroy sensors positioned in the focal point; thus, sensors must be moved very quickly through the focal point to avoid damage from the high intensity laser beam.
To overcome problems inherent in measuring high output intensities, one prior art measuring instrument provides a reflecting needle which is rotated through the laser beam at very high speed. Additionally, other prior art measuring instruments quickly deflect the focused laser beam across a round or slotted aperture by means of rotary mirrors. However, these prior apparatuses are very expensive; furthermore, they are frequently destroyed and for various reasons produce considerable measuring errors.
Another method of measuring high output intensities makes use of the effect of the laser beams on the processed materials. A plastic or wood bar, located at a predetermined angle, is moved very quickly through the focal range, and an inference is made on the focal point position based on the visible geometry of the bar in the zone of exposure to the beam. This method may be inaccurate since the effect of the beam on the bar is influenced by many extraneous factors including unassessed time integration effects. Also, determining the position of the focal point for beams of slender focus is difficult.
What is needed is a process for measuring laser beams with high output intensities wherein the output intensity is sufficiently reduced so that the focal point can be determined safely and accurately with the aid of sensors. Additionally, a device for the application of this process is needed.